The old method: Determining browser type in ASP

In web development it is often important to know which browser is viewing the page you are serving. Traditional ASP applications used the Browser Capabilities object that resides in the \system32\inetsrv\browscap.dll DLL. This object takes the User Agent string that a browser sends to a website and compares it to a list of simple wildcard expressions in the browscap.ini file to determine the type of browser currently requesting a page.

If the expression Mozilla/4.0 (compatible; MSIE 6.0*;*Windows NT 5.1*)* matches the user agent string that the browser has sent, then the browser will be identified as IE 6.0 on the WinXP platform, with the properties defined by the [IE 6.0] section. The file is extensible in the sense that you can add your own properties (such as 'crawler', 'stripper' etc) and add your own browser-sniffing filters for newer browsers such as Firebird and Firefox.

To access this information within an ASP page you would do something like the following:

The main problem with this method is that a new version of a new browser needs a new entry. When IE 7 comes out you will have to update your browscaps.ini file. If you don't then you could be rendering your pages incorrectly to your readers or disabling functionality based on incorrect assumptions.

The new method: Determining browser type in ASP.NET

ASP.NET solves this problem by introducing a more powerful regular expression-based method of determining the browser. Instead of relying on simple wildcard searches, the ASP.NET browser detection uses regular expressions to allow more complex testing and data extraction. An example of the format of the browser-sniffing expressions is given below

This branch specifies that the HTTP_USER_AGENT header sent from the browser should be matched against the <filter> expressions that follow. You will notice the expression differs from the traditional ASP method in that it's possible to extract information from the match and assign the extracted values to browser properties. For example, when the user agent string

Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2) Opera 7.23 [en]

is matched against the expression

Opera[ /](?'version'(?'major'\d+)(?'minor'\.\d+)(?'letters'\w*))

the values 'version', 'major' and 'minor' will be set as '7.23', '7' and '.23' respectively.

Updating browser and platform types

The browser matching regular expressions are contained in the configuration/system.web/browsercaps section of the machine.config file in your \WINDOWS\Microsoft.NET\Framework\v1.1.4322\CONFIG\ directory.

If you wish to add a new platform (eg Windows 2003 or Longhorn) then you can add a new filter under the browsercaps branch below the <use var="HTTP_USER_AGENT" /> directive:

The point here is that future versions of the browser will automatically be detected. As long as your browser-dependant code is of the form if BrowserVersion >= X then ... (and assuming that forward compatibility is maintained for that particular browser) you will be fine.

The Problem, and How You Can Help

With new browsers are constantly being released, and new spiders, crawlers and site-strippers being written it's a difficult job to ensure your config file is up to date. Microsoft has not committed to keeping the machine.config file updated, instead preferring to let cyScape, Inc. do the work. The problem with this is that it isn't being updated, and in fact now seems to have been removed altogether.

Rob Eberhardt at http://slingfive.com/pages/code/browserCaps/ has provided an update for the machine.config file that includes Gecko, Safari and Konqueror browsers, and I've made a small update to demonstrate how to include detection for Windows 2003. There is an excellent browsercap.ini file maintained by Gary Keith at http://www.garykeith.com/browsers/ that lists not only browsers, but all crawlers, spiders and strippers that have been detected. Gary has done an outstanding job.

Rob Eberhardt and I would like to use the combined resources of the CodeProject community, along with the amassed information provided by Gary to start our own CodeProject maintained browsercaps compilation for ASP.NET machine.config files. With your help we can ensure that our browser detection is up to date.

A test page has been provided here. If you have a browser that is mis-reported or not detected then please either let us know or better yet, take the time to determine the expression matches needed and send them in so we can merge them. Better yet, if someone has the time and patience to develop a database driven config file generator then that would automate the process and speed up updates.

For the moment we will do our best to ensure the current file is updated as often as possible. It's over to you.

10 Oct 2005 - Updated XML file with updates for Opera/Gecko/AppleWebKit based browsers. Mainly to properly detect version and other settings according to there websites information. Update provided by Owen Brady

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About the Author

Chris is the Co-founder, Administrator, Architect, Chief Editor and Shameless Hack who wrote and runs The Code Project. He's been programming since 1988 while pretending to be, in various guises, an astrophysicist, mathematician, physicist, hydrologist, geomorphologist, defence intelligence researcher and then, when all that got a bit rough on the nerves, a web developer. He is a Microsoft Visual C++ MVP both globally and for Canada locally.

His programming experience includes C/C++, C#, SQL, MFC, ASP, ASP.NET, and far, far too much FORTRAN. He has worked on PocketPCs, AIX mainframes, Sun workstations, and a CRAY YMP C90 behemoth but finds notebooks take up less desk space.

He dodges, he weaves, and he never gets enough sleep. He is kind to small animals.

Chris was born and bred in Australia but splits his time between Toronto and Melbourne, depending on the weather. For relaxation he is into road cycling, snowboarding, rock climbing, and storm chasing.

Comments and Discussions

it should be noted that the information in this article is relevant for asp.net 1.1 only. in asp.net 2.0 and above browsers are detected with a .browser xml file, located at C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG\Browsers. if you add a new file, it should be registered with Aspnet_regbrowsers.exe. see this MSDN article for details.